The present invention relates to a dual polarisation patch antenna having good isolation between polarisations over a broad bandwidth.
Referring to FIG. 1 a dual polarised antenna of known construction is shown (see for example CHIBA, T., SUZUKI, Y., and MIYANO, N.: xe2x80x9cSuppression of higher modes and cross polarised component for microstrip antennasxe2x80x9d. IEEE AP-S Int. Symposium Antennas and Propagat. Digest, 1982, pp.285-288. The antenna consists of a patch 1 (in dashed outline) fed by probes 2, 3, 4 and 5. Signals of a first polarisation are conveyed between feed point 6 via a first feed path 7 and a second feed path 8. Second feed path 8 includes a half wavelength section 9, making feed path 8 a half wavelength longer than feed path 7. Accordingly, at the central operating frequency, signals from probes 2 and 4 are 180xc2x0 out-of-phase and cancel at probes 3 and 5.
For signals of the second polarisation the feed paths 11 and 12 between feed point 10 and probes 3 and 5 correspond to those described above and so the signals from probes 3 and 5 cancel at probes 2 and 4 at the central operating frequency.
Accordingly, good isolation is achieved between polarisations at the central operating frequency. Referring to FIG. 2 the relationship between the phase of signals supplied to probes 2 and 4 via feed paths 7 and 9 is shown with respect to frequency. It will be seen that at the central operating frequency-fc, the desired 180xc2x0 phase separation is achieved. As shown in FIG. 3 this results in good isolation between the polarisations at the central frequency.
However, the required isolation can only be maintained over a relatively narrow frequency of operation. It would be highly desirable to provide a feed network capable of maintaining isolation greater than 30 db over a wider frequency range.
It is an object of the present invention to provide a dual polarisation patch antenna having improved isolation over a greater frequency range or to at least provide the public with a useful choice.
According to the invention there is provided a dual polarisation patch antenna comprising:
a patch;
a first and second pair of orthogonally disposed probes feeding the patch;
a first feed network for feeding the first pair of probes from a first feed point comprising:
i) a first feed path from the first feed point to a first probe;
ii) a second feed path from the first feed point to a second probe; said second feed path being of a different electrical length to the first feed path such as to cause cancellation of signals from the first and second probes at the second pair of probes; and
iii) a first frequency dependent element provided in the first feed path for maintaining the desired cancellation over a desired frequency range; and
a second feed network for feeding the second pair of probes from a second feed point comprising:
i) a third feed path from the second feed point to a third probe;
ii) a fourth feed path from the second feed point to a fourth probe, said fourth path being of a different electrical length to the first third path such as to cause cancellation of signals from the third and fourth probes at the first pair of probes; and
iii) a second frequency dependent element provided in the third feed path for maintaining the desired cancellation over a desired frequency range.
The frequency dependent element preferably comprises two quarter-wave separated, open half-wavelength stubs. Alternatively, the frequency dependent element may comprise a Schiffman phase shifter.
The feed paths preferably differ by a half-wavelength (at the desired central operating frequency). The feed paths may be of the same physical length. In this case, the difference in electrical length may be achieved by the insertion of a suitable dielectric material adjacent to one of the feed lines, thus reducing the propagation speed in the feed line (and hence increasing the electrical length). However preferably the feed paths are of different physical lengths.